1. Field of the Invention
The present invention relates generally to an engine valve train and more specifically to a variable valve timing arrangement which permits the lift and/or timing of the valves to be selectively varied.
2. Description of the Prior Art
JP-A-63-167016 and JP-A-63-57805 disclose rocker arm arrangements which include a main rocker arm which cooperates with a low speed cam and a sub-rocker arm which cooperates with a high speed cam. The two rocker arms are pivotally mounted on a common rocker arm shaft.
A hydraulically operated connection or interlocking device which enables the main and sub-rocker arms to be selectively locked together, comprises a set of plunger bores which are formed in the rocker arms in a manner to be parallel with and at a predetermined distance from, the axis of the shaft about which the arms are commonly pivotal. By applying a hydraulic pressure to the end or ends of the plungers reciprocally disposed in the bores, the plungers can be induced to move axially and induce the situation wherein two of the plungers will partially enter an adjacent bore and lock the two arms together.
However, these type of arrangement have tended to have suffered from the drawback that as set screws are used to set the valve clearances, depending on engine temperature the amount of tappet noise tends to be excessive. In order to overcome this problem it has been proposed to use hydraulic hydraulic lash adjusters in order to reduce the valve clearance to zero under all modes if engine operation. These lash adjuster have been mounted in the main rocker arm. When the main rocker arm is driven by engagement between the low speed cam and low speed cam follower, the lash adjusters each reduce the clearance between the upper end of the valve stem member and the member of the rocker arm which operatively engages the same.
However, when the mode of engine operation switches to a high speed one and the sub-rocker arm is subsequently locked with the main rocker arm in a manner wherein the main rocker arm is driven by the engagement between the high speed cam and the high speed cam follower (on the sub-rocker arm), due to the inevitable dimensional deviations which occur during manufacture of the valve train components, it sometimes happens that, under interlocked conditions, the high speed cam follower projects slightly above the low speed one (viz., further toward the cam shaft). Thus, as the base circles of the high and low speed cams are ground to an essentially identical diameter, immediately after a change from one speed mode to another, it can occur that the lash adjuster has adjusted its length (elongated) to suit the lift characteristics provided by the "lower" of the two cam followers and is currently too long for the cam follower currently in use. As it takes a finite time for the lash adjuster to adjust its length (i.e. reduce to the appropriate length), there tends to be a period when the valve which is being lifted via contact with the overly elongated lash adjuster, tend to be held slightly open and cannot assume a fully closed condition.